n (British) A small nonmilitary, non-nuclear submarine for exploration.
n (British) A retroactive term used for non-nuclear submarines; nuclear submarines are termed "true submarines".
n (British) A term used primarily by some navies for nuclear submarines, termed "true submersibles", because they cannot retroactively declare that their non-nuclear submarines should be called by a different name.
n (US) A very small "baby" submarine designed for specific localized missions, usually while tethered to a submarine or ship for life support and communications. Slang synonyms: midget-submarine, anchor.